Good day SWCExpat!
That's neat that you found an F350! Those are unusual and of special interest to many of us. If you ever have a chance to put up some photos of it, I know we'd love to check it out!
Everything I've read is in agreement with what Larry said about the factory carburetor on a stock truck 390. I've not read of any advantages to installing anything with greater CFM capacity, as long as the engine's a stock truck engine. As you may or may not know, the 390 FEs put in cars were set up differently and could suck down more CFM than the truck engines could. As I understand it, Ford set up truck FEs primarily for low-RPM torque, and less CFM's required to feed an engine at low RPM like that.
If you'd still like an aftermarket carburetor, it seems that there are guys who like both Edelbrocks and Holleys. Based on the little I know about carburetor sizing though, 750CFM is far too much for a stock truck 390. Of course you could get all kinds of great technical feedback on carburetor sizing if you started a thread specifically on that in the engine section.
It's the opinion of Tom Lucas at FE Specialties (one of the FE authorities) that some carburetors made by Holley are the best fit on FEs, or at least on modified FEs. Yet at the same time, some super experienced and knowledgeable members here like 70_F100 like the Edelbrocks. I think the key with either carburetor type is knowing how to choose the best jets and springs for your application. That's a task that guys like 70_F100 are definitely up to, but beginners like myself are better off getting guidance before making changes.
Everything I've read indicates that headers will provide a significant performance improvement, and that's because the manifolds Ford used on these trucks are too restrictive, or create too much turbulence, or something like that. This said, the stock manifolds fit very neatly under the hood, make less under-hood noise and heat than headers do, and are worlds easier to remove and install. Rich (1972hiboy), a professional mechanic and great guy that lives nearby me, chose to stick with stock manifolds on his 1972 Hi-Boy for these reasons, and his engine compartment looks very tidy that way. His truck also has a tight, clean, low-pitched rumble when it runs. I love the way it sounds.
One of the things that sticks out around the headers most is the big OEM starter. Not only does it stick out, but the heat from the headers can mess it up over time. And when it's time to replace the starter, the header on that side of the engine typically needs to be removed or loosened first. So many guys wind up replacing the OEM starter with a modern, gear-reduction, miniature type starter before putting headers on. These are a lot smaller so they fit under there well, and they're much more heat-resistant. There are several makes that I've read good things about, yet my personal favorite is the RobbMc P/N 2001, because it's 100% USA-made in Nevada, using almost entirely USA-made components. And the guy that makes it, Robb, his a very personable and friendly guy. He's really in to the FE engine and has been active in an FE Web forum (he might still be). When I sent an e-mail to him to ask some questions about his starter, he
personally replied in an unusually warm way with high-quality information. I was sold by that uncommon service!!!
SWCExpat, since I joined FORDification in 2008 I've asked hundreds of questions and received untold hours worth of guidance from top members. All of that guidance, and documentation of it being put in to action, remains available in those threads. A while back I made an index of all the threads to help myself and others find stuff. Maybe you'd be curious to browse through that index some time:
Main #50 Project Thread: 1972 F250 4x2; 8,100lb GVW; FE+T18
Robroy
). Been looking around for something to fix up and keep running for quite a bit longer. Got a '72 F350 with the 390! Who cares about fuel prices, well it won't be my daily driver anyhow. It's a camper special, might not clean her back up for putting a camper on, but to have a good running truck. 
from an EX Chebby guy myself. Don't discount your 2100 two barrel carb, it's a GREAT one...you'll be surprised! No advice on headers for ya, plenty of folks here will chime in with their experience. Recommend them? Only if your current exhaust system is junk. Will headers improve performance? No Doubt!
